1. Field of the Invention
The present invention relates to an apparatus and a method for detecting abnormalities in a secondary air supplier in an internal combustion engine. More particularly, the present invention pertains to detecting abnormalities in accordance with the air-fuel ratio in the exhaust system of the internal combustion engine.
2. Description of the Related Art
Generally, when an internal combustion engine is warming up and/or decelerated, secondary air is supplied to an exhaust system to improve purification of a three-way catalyst provided in the exhaust system.
During warming of the engine, more fuel is supplied to a combustion chamber so that the air-fuel ratio in an intake system is relatively high. When the engine is slowed, unburnt fuel remaining on the inner wall of an intake port is abruptly sent to the combustion chamber under a negative intake pressure. This temporarily increases the air-fuel ratio. In an internal combustion engine equipped with the secondary air supplier, however, secondary air is supplied to the exhaust system during warm-up operations and deceleration, to make the exhaust air-fuel ratio in the exhaust system low.
However, in the event that an abnormality develops such that the secondary air supplier will not work, the exhaust air-fuel ratio during the warm-up operation and deceleration becomes high like the air-fuel ratio in the intake system. Paying attention to this event, various types of apparatuses have been proposed which detect abnormalities in the secondary air supplier.
Such an abnormality detecting apparatus disclosed in, for example, Published Unexamined Japanese patent application No. 68-111256 includes an oxygen sensor in the exhaust system. The oxygen sensor serves to detect the exhaust air-fuel ratio at the time the secondary air supplier is in operation, such as during warm-up and deceleration. When the detected exhaust air-fuel ratio is high, the secondary air supplier is judged as abnormal.
According to the apparatus disclosed in Published Unexamined Japanese patent application No. 63-143362, the average of air-fuel ratio compensating coefficients is acquired based on a value detected by the oxygen sensor. The secondary air supplier is judged as abnormal if the average value is less than or equal to a predetermined value when the secondary air supplier is in operation.
Both detecting apparatuses described above are devised on the premise that the intake air-fuel ratio is slightly greater than the theoretical air-fuel ratio during warm-up and deceleration. They judge the abnormality of the secondary air supplier merely on the determination of whether the exhaust air-fuel ratio is high or low. However, at times, the air-fuel ratio in the intake system may be excessively high due to various conditions, such as a change in revolution speed of the engine. This makes the exhaust air-fuel ratio high even when the secondary air supplier works properly, which may result in an erroneous detection of abnormality. In other words, the conventional detecting apparatus is not so reliable in detecting abnormality.